1. Field of the Invention
This invention pertains generally to rotary shaft seals and, more particularly, to a thermally actuated shutdown seal for a centrifugal liquid pump.
2. Description of the Related Art
In pressurized water nuclear power plants a reactor coolant system is used to transport heat from the reactor core to steam generators for the production of steam. The steam is then used to drive a turbine generator for the production of useful work. The reactor coolant system includes a plurality of separate cooling loops, each connected to the reactor core and containing a steam generator and a reactor coolant pump.
The reactor coolant pump typically is a vertical, single stage, centrifugal pump designed to move large volumes of reactor coolant at high temperatures and pressures, for example, 550° F. (288° C.) and 2,250 psia (155 bar). The pump basically includes three general sections from bottom to top; hydraulic, shaft seal and motor sections. The lower hydraulic section includes an impeller mounted on the lower end of the pump shaft which is operable within the pump casing to pump reactor coolant about the respective loop. The upper motor section includes a motor which is coupled to drive the pump shaft. The middle shaft seal section includes three tandem seal assemblies; lower primary (number 1 seal), middle secondary, and upper tertiary seal assemblies. The seal assemblies are located concentric to, and near the top end of, the pump shaft and their combined purpose is to provide for minimal reactor coolant leakage along the pump shaft to the containment atmosphere during normal operating conditions. Representative examples of pump shaft seal assemblies known in the prior art are described in U.S. Pat. Nos. 3,522,948; 3,529,838; 3,632,117; 3,720,222 and 4,275,891.
The pump shaft seal assemblies which mechanically seal the interface between the stationary pump pressure boundary and the rotating shaft, must be capable of containing the high system pressure (approximately 2,250 psi (155 bar)) without excessive leakage. The tandem arrangement of three seal assemblies is used to break down the pressure in stages. These three mechanical pump seal assemblies are controlled-leakage seals which in operation allow a minimal amount of controlled leakage at each stage while preventing excessive leakage of reactor coolant from the primary coolant system to respective seal leakoff ports.
The pump seal assemblies are normally maintained at temperatures well below those of the primary coolant system, either through injection of cool fluid at the seal assemblies or through the use of a heat exchanger which cools the primary fluid before it reaches the seal assemblies. Theorized failure of these systems may expose the seal assemblies to high temperatures which will likely cause the controlled leakage of the seal assemblies to increase dramatically. When the cause of the loss of all fuel cooling is due to losing all AC power, the seal leakoff has no means of returning to the coolant system without electricity to power the makeup pumps. Controlled leakage without the means of makeup could hypothetically lead to reactor coolant uncovering the reactor core and subsequent core damage.
Consequently, a need exists for an effective way to back up the standard seal assemblies in the event of a coincidental loss of all fuel cooling and loss of makeup pumping. Furthermore, preferably such a backup seal should be operable upon loss of power or other cause for the loss of makeup pumping capacity to substantially seal the shaft from leakage.